Beginning with the discoveries of giant magnetoresistance (GMR) and later tunneling magnetoresistance (TMR), there has been tremendous effort towards understanding the dynamics of the electron spin and its potential use in electronic circuits, leading to the rapidly developing field of spintronics. GMR and TMR provide means for all-electrical readout in magnetic sensors and memory devices based on ferromagnet/spacer/ferromagnet stacks. However, although four distinct stable magnetic states are possible, (, , , ) (the arrows indicate the magnetization directions of the two layers), magnetoresistive sensing can only distinguish between two resistance states, parallel and antiparallel. Hence, a conventional magnetoresistive stack has one magnetic layer that remains fixed and serves as a reference layer, rather than to store a bit directly.
Switching of memory cells, such as magnetic tunnel junctions, has conventionally been accomplished by injecting a spin current from the fixed layer to the free layer or vice-versa to exert spin-transfer torque on the free layer. Recently, however, it has been found that spin currents can be more efficiently injected by utilizing the spin Hall effect (SHE) in a nonmagnetic metal (NM) layer 120 adjacent a ferromagnetic (FM) free layer 110 as shown in FIG. 1A. An in-plane charge current Jcharge near the NM/FM interface 130 leads to a vertical spin current Jspin that exerts spin-orbit torques that can be used to efficiently control the magnetization. This has been used to achieve spin-orbit torque switching of the free layer in magnetic tunnel junctions, efficient current-induced domain wall motion, and control of magnetic elements in spin-logic devices.
Recently, the SHE has been shown to also lead to new transport phenomena such as the spin Hall magnetoresistance. Even more recently, a related magnetoresistance effect has been reported in NM/FM bilayers, resulting from the interaction of the current-induced interface spin accumulation due to the spin Hall effect, and the magnetization. This so-called unidirectional spin Hall magnetoresistance (USMR) is a nonlinear and nonreciprocal effect that modulates the longitudinal resistivity depending on the component of the in-plane magnetization vector perpendicular to the current injection direction (|my|).
The USMR allows for the detection of in-plane magnetization reversal along the axis collinear with the interface spin accumulation without requiring an auxiliary magnetic layer, as depicted in the plot in FIG. 1B. Although this effect can be rather small, e.g., about 0.002%-0.005% of the total resistance in NM/FM bilayers for a current density of j=1011 A/m2, the chiral property of the USMR distinguishes it from other linear magnetoresistance effects, such as the spin Hall magnetoresistance and anisotropic magnetoresistance, which are both current-independent and proportional to |m|2.